Optical beam scanners are commonly made using one or more mechanical devices to rotate and/or translate an optical element, such as a mirror, that scans, i.e. produces a controlled deflection of the path of an optical beam. Optical elements are those elements that come in contact with the optical beam and may influence the subsequent optical path of the optical beam such as mirrors, lenses, prisms, diffraction gratings, optical conduits, windows, gradient index of refraction materials, acousto-optic and electro-optic materials, and various liquids and gasses including the ocean and the atmosphere. Herein, optical beams, beams, and light, refer not only to waves in the visible spectrum but also to any electromagnetic waves.
An aim of optical scanners is to produce the beam deflections in a controlled manner whereby the accuracy of the actual positions of the beam over the scan as compared with the desired positions of the beam over the scan, is not compromised, or at least relatively uncompromised. One goal of the designer wishing to employ an optical scanner in an application that requires the controlled deflection of the beam, is to understand the limitations of optical scanners, i.e. that it is essentially not possible to make an optical scanner with perfect accuracy; and for the designer to therefore allow some tolerable error in the desired positions. A closed-loop servo device is typically used to compare the actual positions of the scanner with the desired positions and then to make appropriate corrections to the scanner position to minimize the errors thereby controlling the scan.
Mirrors rotating about an axis defined by a spindle, or torsion bar, are called galvanometer scanners. In the galvanometer scanner, a motor device is typically used employing a magnetic field and magnets to rotate a spindle or twist a torsion bar. A capacitance sensor or an optical sensor is typically used to measure a rotation angle of the spindle or torsion bar. It is useful, in many galvanometer scanner applications, to use a servo to control the angle of the desired beam deflection using the sensor information for the actual beam deflection. Two galvanometer scanners are typically used to deflect a beam in two dimensions.
More recently MEMS (micro electro-mechanical systems) mirrors are used to deflect a beam in two dimensions resulting in a compound angle (for example U.S. Pat. No. 6,538,799) with electric field and/or magnetic field actuators used to induce motion in the mirrors. Again capacitance or optical sensors are used to determine the actual position of the mirror defined as the compound angle of the mirror normal vector.
Mirror based optical scanners work best when the beam that is to be scanned has a small angular spread. The angular spreading of the beam can be expressed as the divergence, θ1/2 (where θ1/2 is the half angle of the beam divergence and is defined as the ratio of the beam radius to the effective focal length). As shown in FIG. 1, a substantially large range of positions on the screen 140, may be scanned using the rotating scan mirror 150, with this small divergence beam 120, which is made by lens 130, collecting the light from a light source 110.
It would be desirable in many applications to have a mirror based optical scanner than can address a substantially large range of positions, that could be used with large divergence beams. However, as shown in FIG. 2 with a large divergence beam 220, which is formed from the light source 210, the performance of the rotating scan mirror 250, is limited in scan angle as observed at the screen 240.
In both FIG. 1 and FIG. 2, the scanned beam is not in-line with the original beam. In order to have the scanned beam in-line at least in the same direction with the original beam, another either moving or stationary mirror 350, is required, and this is shown in FIG. 3. This optical configuration makes it difficult in some applications to scan an array of beams that may be next to each other. It would be desirable to be able to have the scanner in-line with the original beams so that an array of beams that are next to each other could be scanned over a controlled and over a substantial range of positions, by an array of scanners.
Another desirable feature of a scanner is the ability to perform the scan over a substantially large range of positions, in two dimensions with a single mirror device. Typically however, galvanometer based scanners require two scanner mirrors and two galvanometers, for example, one rotation for the vertical axis and one rotation for the horizontal axis. While one mirror may be used in principle to scan in two dimensions, there are problems with the actuation, the scan angle, and the speed of the scan, that limit performance of true two-dimensional single-mirror galvanometer based scanners.
While the MEMS mirrors described above can overcome the compound scan angle difficulty another difficulty is encountered when a substantially large range of positions of a compound rotation angle is induced in a single mirror. That problem is the difficulty of determining the position of the mirror since measurement of one rotation axis by, say, a capacitance sensor or optical sensor, is influenced by the motion in the other rotation axis. So it is desirable to have a way of determining the compound pointing angle direction whereby the two angular sensors are substantially independent of each other.
Another desirable feature of a scanner is having the scanner comprised of a single optical element that may reliably perform functions of both torsion bar and optical element (however again, the MEMS mirrors described can overcome this difficulty, albeit with four torsion bars whereas fewer torsion bars, e.g. one torsion bar, would be desirable).
Another desirable feature of a scanner is the ability to perform the scan over a substantially large range of positions, in two or more dimensions with a single device. Typically however, galvanometer based scanners require two scanner mirrors and two galvanometers, for example, one rotation for the vertical y-axis and one rotation for the horizontal x-axis; and in addition a focussing lens is used for the z-axis. While this configuration may be used in principle to scan in three dimensions, there are problems with the actuation, the scan angle, and the speed of the scan, which limit performance of existing three-dimensional scanners.